1. Field of the Invention
The present invention relates to a torch for gas shielded arc welding using a consumable electrode. More particularly, the invention pertains to a welding torch used therefor and well suited for titanium welding.
2. Description of the Related Art
Articles of stainless steel are bonded to one another by means of welding such as arc welding with a covered electrode, TIG (tungsten inert gas) welding, MIG (metal inert gas) welding, electron beam welding or the like. Articles of titanium which are more enriched in activity than stainless steel articles are bonded to one another by welding such as TIG welding, MIG welding, electron beam welding or the like.
In the case of welding with a covered electrode, It Is impossible to completely prevent ambient air (oxygen) from mixing in a deposited metal, whereby the deposited metal is made brittle. As an alternative, it is preferred to employ TIG welding by which welding is carried out with a tungsten (W) electrode, while the electrode is shielded from outside air with a shielding gas.
However, the above-mentioned TIG welding is of limited productivity because of a limited current load on a tungsten electrode. Thus, it becomes necessary to employ gas shielded arc welding in which a consumable electrode is used for the reason that a current load can surpassingly be enhanced when welding is performed with a melting electrode. The aforesaid gas shielded arc welding using a consumable electrode is exemplified by MIG welding, MAG (metal active gas) welding and CO2 (carbon dioxide) gas shielded arc welding. In the MIG welding, an inert gas such as argon gas is used as a shielding gas. In the MAG welding, use is made, as a shielding gas, of a mixed gas in which an inert gas is mixed with a slight amount of oxygen gas or carbon dioxide gas. In CO2 gas shielded arc welding, carbon dioxide gas is used. Since argon gas is expensive while carbon dioxide gas is inexpensive, the MAG welding is advantageous, and CO2 gas shielded arc welding is further advantageous from the aspect of production cost. The MIG welding, MAG welding and CO2 gas shielded arc welding are similar to one another with respect to electrode melting and interception of outside air with a shielding gas. It being so, the term xe2x80x9cgas shielded arc welding using a consumable electrodexe2x80x9d is used as such, or as the case may be, referred simply as xe2x80x9cMIG weldingxe2x80x9d hereinafter.
MIG welding has recently been advanced in its improvement and commercialization. Thus, proposals were made for instance, on a torch structure in Japanese Patent Laid-Open Publication No. HEI-10-128546 entitled xe2x80x9cTORCH FOR ARC WELDINGxe2x80x9d, and on torch cleaning technology in Japanese Patent Laid-Open Publication No. HEI-9-085448 entitled xe2x80x9cTORCH FOR WELDINGxe2x80x9d. Nevertheless, it has been proved that even by the use of the above-mentioned welding torch, the fundamental problems as described hereunder still remain unsolved.
Hereinafter, description will be given as to the principle of a conventional torch with reference to FIG. 4 hereof. Torch 100 for MIG welding is a widely used and typical one comprising: a torch body 104 equipped with a wire guide cylinder 103 which guides a welding wire 102 that is fed downward in FIG. 4 at a prescribed feed rate by means of feed rollers 101, 101; a gas nozzle 105 fitted to the torch body 104 in such a manner as to surround the wire guide cylinder 103; and a shielding gas passageway 107 for feeding a shielding gas in a gas passageway 106 formed between the wire guide cylinder 103 and the gas nozzle 105.
In this arrangement, when welding voltage is applied between the welding wire 102 and a weld base metal 110, discharged arcs 111 are scattered and produce arc heat therebetween, by which heat the weld base metal 110 as well as the welding wire 102 melt away. In the Figure, reference numerals 112 and 113 denote a molten pool and deposited metal, respectively.
At this time, an inert gas is fed in the gas passageway 106 through the shielding gas passageway 107, and is ejected through the lower end of the gas passageway 106, namely the opening at the lower end of the gas nozzle 105 as shown by arrows {circle around (1)}, {circle around (1)}, that the discharge arcs 111 are enclosed by the inert gas thus ejected so as to prevent outside air (oxygen) from reaching the molten pool 112. The interception of outside air with a gas is termed xe2x80x9cgas shieldingxe2x80x9d, and the gas used therefor is termed xe2x80x9cshielding gasxe2x80x9d.
Next, reference is made to FIG. 5 illustrating a problem with the conventional MIG welding. When the MIG welding torch 100 is moved from front to back, the weld base metal 110 as illustrated in FIG. 5 causes a surface oxide 114 although being in a slight amount to sometimes generate in the vicinity of the deposited metal. The surface oxide 114 is generated in stainless steel, and inter alia markedly in titanium which is an active metal more prone to combine with oxygen.
It being so, the discharged arcs 111 rock towards the surface oxide 114. The rocking phenomenon, which is termed xe2x80x9cwandering phenomenonxe2x80x9d, instabilizes welding itself when arises. When the feed rate of the inert gas is increased as a countermeasure against the generation of the surface oxide 114, the inert gas constitutes so-called xe2x80x9cturbulent flowxe2x80x9d. The resultant turbulent flow catches up circumferential gases, thereby causing outside air to be caught up therein with the result that the increased feed rate thereof is not recognized as a considerably effective countermeasure.
With regard to the aforesaid wandering phenomenon, the MAG welding can be said to be an improved method over the MIG welding from the viewpoint that a slight amount (several percent) of oxygen gas or carbon dioxide gas is mixed in the inert gas in order to suppress the phenomenon.
In the MAG, welding argon gas is mixed with, for example, 2.0% to 3.0% of oxygen gas. The amount of the oxygen gas, hen being less than. 2.0%, causes the wandering phenomenon, whereas the amount, when being more than 3.0%, brings about the unfavorable result that the surface oxide 114 becomes more prone to be generated. It is therefore obliged to strictly regulate the mixing proportion of the gas to be added thereto (oxygen gas). For this reason, the burden on a welding worker is greatly increased, or it becomes necessary to adopt a mixing unit equipped with a precise control unit. Accordingly, it follows that the MAG welding results in high welding cost as compared with the MIG welding.
It is therefore an object of the invention to provide a technique which enables welding technically comparable to MAG welding at a welding cost competitive with MIG welding, and also a technique which enhances the quality of welding, in the case of welding stainless steel and titanium that are more prone to combine with oxygen.
To attain the foregoing object, according to the present invention, there is provided a torch for gas shielded arc welding using a consumable electrode, comprising: a torch body having a wire guide cylinder for guiding a welding wire; an internal cylinder fitted to the torch body in such a manner as to surround the wire guide cylinder; an external cylinder fitted to the internal cylinder in such a manner as to surround the internal cylinder; a first shielding gas feed route for feeding a shielding gas in a first gas passageway formed between the wire guide cylinder and the internal cylinder; and a second shielding gas feed route for feeding the shielding gas in a second gas passageway formed between the internal cylinder and the external cylinder.
The above-mentioned torch has a tip configuration of double pipe structure composed of an internal cylinder and an external cylinder, and a shielding gas is fed in a first gas passageway formed in the internal cylinder, and in a second gas passageway formed between the internal cylinder and the external cylinder. Thus, primary interception of outside air is put into practice with a second shielding gas layer blown off through the second gas passageway arranged on the outer side, and complete interception of outside air is put into practice with the first shielding gas layer blown off through the first gas passageway arranged on the inner side.
Preferably, the internal cylinder is composed of a large diameter portion which is fitted to the torch body, and a small diameter portion at an end which is connected to the large diameter portion via a tapered tubular portion. It is made possible to increase the flow velocity of the shielding gas blown off through the internal cylinder by such construction that the small diameter portion is formed at an end and also to fit the cylinder to an existing torch body by forming the large diameter portion at the root portion thereof.
Also preferably, a porous member is allowed to intervene between the internal cylinder and external cylinder (second gas passageway) so as to rectify the shielding gas passing through the second gas passageway. The porous member is exemplified by a stainless steel wire mesh, and filled in the second gas passageway. The shielding gas is decelerated and dispersed through the stainless steel wire mesh, and hence is blown off at a low velocity in a rectified state, thereby eliminating the anxiety about catching up outside air.
Also preferably, the tip of the external cylinder recedes from the tip of the internal cylinder so as to direct the shielding gas passing through the second gas passageway toward radial outside, and enable to sweep away penetrating outside air toward radial outside.